The topics covered in this chapter can be summarized as follows:

Section Summary
5.1 Weathering Rocks weather when they are exposed to surface conditions, which in most case are quite different from those at which they formed. The main processes of mechanical weathering include exfoliation, freeze-thaw, salt crystallization, and the effects of plant growth. Chemical weathering takes place when minerals within rocks are not stable in their existing environment. Some of the important chemical weathering processes are hydrolysis of silicate minerals to form clay minerals, oxidation of iron in silicate and other minerals to form iron oxide minerals, and dissolution of calcite.
5.2 The Products of Weathering and Erosion The main products of weathering and erosion are grains of quartz (because quartz is resistant to chemical weathering), clay minerals, iron oxide minerals, rock fragments, and a wide range of ions in solution. Without weathering, there would not be sediment available to eventually form sedimentary rocks!
5.3 Clastic Sedimentary Rocks Sedimentary clasts are classified based on their size, and variations in clast size and shape have important implications for transportation and deposition. Clastic sedimentary rocks are classified based on their grain size and composition. Clast size, sorting, composition, and shape are important features that allow us to differentiate clastic rocks and understand the processes that took place during their deposition.
5.4 Chemical Sedimentary Rocks Chemical sedimentary rocks form from ions that were transported in solution, and then converted into minerals by biological and/or chemical processes. The most common chemical rock, limestone, typically forms in shallow tropical environments, where biological activity is a very important factor. Names of limestones can be modified with textural terms like crystalline, oolitic, or fossiliferous. Chert is a deep-ocean sedimentary rocks. Evaporites (rock salt and rock gypsum) form where the water of lakes and inland seas becomes supersaturated due to evaporation. Coal forms in swamps from decaying plant remains.
5.5 Depositional Environments and Sedimentary Basins There is a wide range of depositional environments, both on land (glaciers, lakes, rivers, etc.) and in the ocean (deltas, reefs, shelves, and the deep-ocean floor). In order to be preserved, sediments must accumulate in long-lasting sedimentary basins, most of which form through plate tectonic processes.
Lab 5 Exercises The best way to learn rock identification is to practice by examining the samples in your Rock Kit 1 and 2. The first step when examining a sedimentary rock is to identify the texture. Clastic sedimentary rocks have clastic textures, and are classified based on grain size, and for sandstones, also by composition. Chemical sedimentary rocks are often monomineralic and are classified based on composition. Chemical sedimentary rocks can have a range of textures (crystalline, clastic, bioclastic, fossiliferous, oolitic, and amorphous). Knowing the diagnostic properties of the main minerals that form chemical sedimentary rocks will help you correctly identify the rock. Just as with mineral samples, different samples of the same rock may not always look exactly the same (e.g., tan versus blue-grey crystalline limestone), but they can always be identified by closely examining the mineral composition and texture.


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A Practical Guide to Introductory Geology Copyright © 2020 by Siobhan McGoldrick is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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